With the development of multimedia applications, it has become common in recent years to handle information of all sorts of media such as audio, video and text in an integrated manner. In doing so, it becomes possible to handle media integrally by digitalizing all the media. However, since digitalized pictures have an enormous amount of data, information compression techniques are of absolute necessity for their storage and transmission. On the other hand, in order to interoperate compressed picture data, standardization of compression techniques is also important. Standards on picture compression techniques include H.261 and H. 263 recommended by ITU-T (International Telecommunication Union Telecommunication Standardization Sector), and MPEG (Moving Picture Experts Group)-1, MPEG-2 and MPEG-4 of ISO (International Organization for Standardization).
FIG. 1 is a block diagram showing a structure of a conventional picture coding apparatus 100. Note that, in the present invention, the unit consisting of one sheet of image is referred to as a picture. In an interlace image signal, a picture means a field or a frame, and in a progressive image signal, a picture means a frame.
The picture coding apparatus 100 includes a difference calculator 101, a picture coding unit 102, a variable length coding unit 103, a picture decoding unit 104, an adder 105, an inter pixel filter 106, a picture memory 107, an inter picture predicting unit 108, and an inter picture prediction estimating unit 109. The difference calculator 101 subtracts a predictive picture inputted to a minus input terminal from an input picture inputted to a plus input terminal so as to output the differential picture. The picture coding unit 102 codes the inputted differential picture. For example, the picture coding unit 102 codes the inputted data by performing frequency transformation of it using DCT or the like and quantizing the frequency data as the transformation result. The variable length coding unit 103 performs variable length coding of the coded differential picture and predictive parameters from the inter picture prediction estimating unit 109, adds relevant data such as a header describing information relevant to the resulting coded data to it, formats it into an output coded bit stream, and outputs it outside the picture coding apparatus 100. The picture decoding unit 104 decodes the differential picture coded by the picture coding unit 102, by performing processing inverse to the coding by the picture coding unit 102 for the differential picture. For example, after performing inverse quantization of the coded differential picture, the picture decoding unit 104 performs inverse frequency transformation such as inverse DCT to decode the difference between the input picture and the predictive picture. The adder 105 adds the decoded differential picture and the predictive picture to decode the input picture. The inter pixel filter 106 performs filtering such as suppressing coding noise in the high frequency components of the decoded input picture. The picture memory 107 holds the picture data of at least one picture among the pictures decoded by the adder 105 as a reference picture. The inter picture predicting unit 108 reads out a predictive picture from the reference picture in the picture memory 107 based on the prediction result obtained by the inter picture prediction estimating unit 109. The inter picture prediction estimating unit 109 derives a predictive parameter PredParam that is the amount of change in motion of the input picture from the reference picture.
More specifically, picture data Img is inputted to the picture coding apparatus 100 from outside. The picture data Img is inputted to the plus input terminal of the difference calculator 101. The difference calculator 101 calculates the difference between the pixel values of this picture data Img and the predictive picture data Pred inputted to the minus input terminal to output the result as differential picture data Res. This predictive picture data Pred is obtained in the following manner. First, reference picture data Ref, that is an image of an already coded picture, and is once coded and then decoded to be an image for one picture, is stored in the picture memory 107. Next, from this reference picture data Ref, data representing an image corresponding to each block in the inputted picture data Img is extracted based on the predictive parameter PredParam. The data representing this image of each block is the predictive picture data Pred. The picture coding apparatus 100 stores several sheets of coded pictures as reference picture data Ref for prediction in the picture memory 107, and the inter picture predicting unit 108 generates predictive picture data Pred from the reference picture data Ref stored in the picture memory 107. The inter picture prediction estimating unit 109 obtains predictive parameter data PredParam used for prediction (for instance, motion vector information used in the MPEG picture coding method, and the like) from the input picture data Img and the reference picture data Ref. Note that pixel values of a predictive picture shall be “0” in the case of intra picture coding.
The picture coding unit 102 codes differential picture data Res, and outputs it as coded differential picture data CodedRes. The picture decoding unit 104 decodes the coded differential picture data CodedRes and outputs it as decoded differential picture data ReconRes in order to use it as a reference picture for inter picture prediction. The pixel values indicated by this decoded differential picture data ReconRes and pixel values indicated by the predictive picture data Pred are added by the adder 105 and outputted as decoded picture data Recon. The inter pixel filter 106 performs filter operation processing for the decoded picture data Recon, and stores it as filtered decoded picture data FilteredImg in the picture memory 107.
The inter pixel filter 106 has the effect of reducing coding noise of decoded picture data Recon and improving prediction efficiency if the picture is used as a reference picture. As an example of the inter pixel filter 106, there is an H.261 loop filter recommended by ITU-T. The filtered decoded picture data FilteredImg which has been performed of the inter pixel filter operation by the inter pixel filter 106 is stored in the picture memory 107, and used as a reference picture when the following pictures are coded. The variable length coding unit 103 performs variable length coding of the coded differential picture data CodedRes and the predictive parameter data PredParam, and puts them together into one coded data Bitstream to output the result outside the picture coding apparatus 100. FIG. 2 is a block diagram showing the structure of a conventional picture decoding apparatus 200. The picture decoding apparatus 200 includes a variable length decoding unit 201, a picture decoding unit 202, an adder 203, an inter pixel filter 204, a picture memory 205 and an inter picture predicting unit 206. The coded data Bitstream is inputted to the picture decoding apparatus 200 from outside. The variable length decoding unit 201 performs variable length decoding of the inputted coded data Bitstream, and separates it into coded differential picture data CodedRes and predictive parameter data PredParam. The picture decoding unit 202 decodes the coded differential picture data CodedRes and outputs it as decoded differential picture data ReconRes. Note that a picture which has been referred to by a picture outputted as the decoded differential picture data ReconRes, that is, a picture corresponding to the reference picture data Ref in the picture coding apparatus 100, has been already decoded and stored in the picture memory 107. Therefore, the inter picture predicting unit 206 generates predictive picture data Pred from the reference picture data Ref based on the predictive parameter data PredParam. The adder 203 adds the predictive picture data Pred and the decoded differential picture data ReconRes to output the result as decoded picture data Recon. The inter pixel filter 204 performs inter pixel filter operation of the decoded picture data Recon, and outputs the result as filtered decoded picture data FilteredImg outside the picture decoding apparatus 200. The outside of the picture decoding apparatus 200 means a display apparatus such as a television. And, the filtered decoded picture data FilteredImg is stored in the picture memory 205 and referred to as reference picture data Ref for the following pictures.
However, if considering the case of transmitting moving pictures via a mobile phone or the like, it is desired to minimize power consumption of a mobile apparatus as a whole in order to make available hours per charge longer, so it is not desirable for the apparatus to have high processing capability which requires a larger amount of power for picture processing. Therefore, in using a mobile apparatus which can use only an operating unit with low processing capability for picture processing, there is sometimes the case where it can use only an inter pixel filter which is required of a small processing load. Meanwhile, it is sometimes requested to use a high-performance inter pixel filter for some applications that strongly require transmission of high quality moving pictures and high coding efficiency, even if an operating unit is required of larger processing load. If a coding method is capable of responding to such requirements flexibly, it is useful with the expanding range of applications to a picture coding apparatus and picture decoding apparatus according to such a coding method and decoding method.
The present invention has been conceived in view of these conventional problems, and aims at providing a picture coding apparatus that uses an inter pixel filter selectively depending on various situations so as to generate coded data, and a picture decoding apparatus that decodes the coded data.